The present invention pertains to electret fibers and electret filter media made of fibers such as melt-blown polymer microfibers and methods of making electret fibers and filters. More specifically, the invention pertains to electret fibers containing a low level of extractable hydrocarbon material. The level of extractable hydrocarbon material is a function of the polymer selected and the processing conditions employed to make the fibers and filters.
Electret articles comprise a dielectric material exhibiting a persistent or quasi-permanent electrical charge. See G. M. Sessler, Electrets, Springer Verlag, N.Y. (1987). The articles are commonly used in the form of fibrous filtering webs for applications, and processes for making electret nonwoven fibrous filter webs are very well known. For example, nonwoven webs can be made from polymers using melt-blowing techniques, such as those described in Van Wente, xe2x80x9cSuperfine Thermoplastic Fibers,xe2x80x9d Ind. Eng. Chem., vol. 48, pp. 1342-46, (1956), and an electric charge can be imparted in the web using various techniques. (See U.S. Pat. Nos. 4,215,682; 4,588,537; 5,411,576 and 5,472,481; 5,645,627; 5,496,507; and WO 97/07272).
Because of the importance of air filtration and the desirable properties that electret filter webs have shown in filter applications, considerable efforts have been devoted to improving the performance of fibrous electret filters. The above-cited patents reflect some of the work that has been reported to improve electret filter performance, and what follows is a brief summary of these contributions.
Kubik and Davis in U.S. Pat. No. 4,215,682 imparted an electric charge in melt-blown fibers by bombarding the fibers with electrically charged particles as the fibers issued from a die orifice.
Klaase et al. in U.S. Pat. No. 4,588,537 injected charge into an electret filter using a corona treatment.
Jones et al. in U.S. Pat. Nos. 5,411,576 and 5,472,481, disclose electret filters that are made by extruding a blend of polymer with a melt-processable fluorochemical in a microfibrous web. The resulting web is annealed and corona treated.
Lifshutz et al. in U.S. Pat. No. 5,645,627 (WO 96/26783) makes electret filters by extruding a blend of polymer with a fatty acid amide or a fluorochemical oxazolidinone or a mixture of these, in a microfibrous web, followed by annealing and corona treating the resulting web.
Angadjivand et al. in U.S. Pat. No. 5,496,507 indicate that impinging water droplets onto a nonwoven microfiber web imparts a charge to the web.
Rousseau et al. in WO 97/07272 disclose electret filters that are made by extruding blends of a polymer with a fluorochemical or organic triazine compound into a microfiber web, followed by impinging water droplets onto the web. This publication indicates that use of these additives results in improved charge when the web has been impinged by water droplets.
Although the above documents disclose a variety of methods for improving electret filter performance, the previous efforts have nonetheless left room for further contributions and the invention described below is yet another discovery directed toward the ongoing effort of establishing better electret fibers and filters.
The present invention provides electret fibers and filters comprised of a polymeric material and a fluorochemical additive. The fibers have less than about 3 weight percent of extractable hydrocarbon material based on the weight of the fibers. The level of extractable hydrocarbon material is measured by extracting the fibers in CHCl3 for 10 minutes at room temperature and measuring the amount of material that has dissolved out from the fibers.
The inventive fibers having less than about 3 weight percent of extractable hydrocarbon material can be made by a process of blending a polymer with a fluorochemical additive compound, extruding the blend at a temperature maintained below 290xc2x0 C. to form extruded fibers, and annealing and charging the extruded fibers.
The present inventors have discovered that there is a correlation between the level of extractable hydrocarbon material found in an electret filter and the filter""s loading performance. Surprisingly, the inventors discovered that the lower the level of extractable hydrocarbons in the extruded web, the better the loading performance of the web (loading performance involves a filter""s ability to remove an oily aerosol from a gas stream and is described in detail in the Examples section). An electret filter web""s performance thus can be predicted by measuring the level of extractable hydrocarbons.
The extractable hydrocarbon level in the extruded web is a function of polymer type and processing conditions used to make the web. Selecting the correct polymer type can be important to achieve a low level of extractable hydrocarbon material in the filter web""s fibers. Harsh processing conditions such as peroxide use and high extrusion temperatures should be avoided because they can increase extractable hydrocarbons and cause a corresponding decrease in loading performance. Thus, controlling the foregoing parameters can result in a filter web exhibiting improved loading performance.
The electret fibers and filters of the present invention find multiple uses including, but not limited to, use in: respirators such as face masks, home and industrial air-conditioners, air cleaners, vacuum cleaners, medical and other air line filters, and air conditioning systems in vehicles and electronic equipment such as computers and disk drives.